Crewel embroidery is the Mother Craft of British, and perhaps even all European embroidery. A rough definition is "embroidery done on linen with fine wool thread", it comprises so many stitches and ideas that it might even be considered the Mother of all Western Needlecraft.
The main idea is that you're making a picture or design or message with wool yarn spun into thread on coarsely woven linen fabric. Some people nowadays use DMC cotton. Suit yourself. That said, it's an amazing high art, which I've likened to rock guitar.
Rhythm stitches in crewel are yes, called "fills". These are the barre chords, the simple strumming patterns that you have to execute flawlessly, over and over and over. This is where you get bargello, needlepoint, cross-stitch, chain and satin fills. Yes, you can make a whole piece out of them, the way you can learn to play "Johnny Be Good" with regular chords,and still make the evening for a modest audience. But that's not the end of the story...
Just as there are rhythm stitches in guitar, there are lead notes in crewel. The only way this breaks down is that while there are many, many chords, but only 12 tones in a scale. In crewel, there are only a dozen or so ways to fill a space, there is a dauntingly many ways to make a single stitch stand out and be special. There are single stitches that will fill almost every niche. Want to stitch an ear of grain? There's a specialized stitch for just that. Need a large or even tiny dot? There's a stitch for that. Want to evoke a field of flowers? You got it. Whether you want to weave your own fabric to lie on top of your work (and pad and embroider that, it's called stumpwork), or simply want to 'draw' a picture, the way you might use a pencil, there's a stitch for that. (With that, and a satin fill, you can stitch your own Bayeux Tapestry!) A skilled seamstress knows at least thirty or forty of these. My mom knew about sixty!
A sampler, therefore, is somewhat of a practice session, rather than a public performance. Most 'real' samplers aren't virtuosic displays, meant to be framed and admired, but reference pieces, for future use. Then, too, any fine-muscular skill has to be kept in trim, and as such is awfully calming...
The other variable is the fabric.It's bitchy. Just when you have the absolute point-perfect place for your stitch....no. It's always going to be a few micrometers away from perfection. Deal with it. It's the roughness that makes it special. Still, it's kind of like venipuncture, trying to figure out whether you dip on what side of a given thread you hit with a needle...which is also part of music, right?